1. Field Of The Invention
The present invention generally relates to a refrigerant compressor and, more particularly, to a slant plate type compressor, such as a wobble plate type compressor, with a variable displacement mechanism suitable for use in an automotive air conditioning system.
2. Description Of The Prior Art
A wobble plate type compressor with a variable displacement mechanism suitable for use in an automotive air conditioning system is disclosed in U.S. Pat. No. 3,861,829 issued to Roberts et al. As disclosed therein, the compression ratio of the compressor may be controlled by changing the slant angle of the sloping surface of the wobble plate. The slant angle of the wobble plate is adjusted so as to maintain a constant suction pressure in response to changes in the pressure differential between the suction chamber and the crank chamber. The difference in pressure between the suction chamber and the crank chamber is generated by a valve control mechanism which controls communication between the suction chamber and the crank chamber. The valve control mechanism varies the crank chamber pressure in response to the suction chamber pressure. In this prior art technique, the crank chamber pressure which generates the changes in the slant angle of the wobble plate is obtained by compressed refrigerant gas which passes through a gap between the cylinder and the piston. This gap is due to the use of a cast iron piston ring disposed at an outer peripheral surface of an aluminum alloy piston housed within a cast iron lined cylinder.
Recently, however, cylinder blocks have been formed of aluminum alloys in order to reduce the weight of the compressor. A seamless piston ring made of polytetrafluoroethylene resin has been disposed at an outer peripheral surface of the piston to prevent wear of both the piston and the cylinder block due to friction therebetween. However, the piston rings enlarge due to swelling during use, thereby significantly reducing the amount of compressed refrigerant gas which is passed to the crank chamber. It is therefore difficult to obtain a crank chamber pressure which satisfactorily generates appropriate changes in the slant angle of the wobble plate. This difficulty is compounded with the use of two of the above-mentioned piston rings, one of which is disposed at an upper portion of the piston and the other of which is disposed at a lower portion of the piston so as to prevent direct contact between the piston and an inner surface of the cylinder.
To overcome this difficulty, U.S. patent application Ser. No. 068,580 filed July 1, 1987 discloses a polytetrafluoroethylene resin made piston ring having a plurality of axial cut-out portions. However, the depth of axial cut-out portions of the piston ring is gradually reduced due to the swelling of the piston ring. Again, it becomes difficult to generate a crank chamber pressure which satisfactorily generates the appropriate changes in the slant angle of the wobble plate throughout the useful life of the compressor.
U.S. Pat. No. 4,428,718 discloses a valve control mechanism responsive to both suction and discharge pressures which controls communication of these pressures with the compressor crank chamber to control compressor displacement. However, extremely precise machining of the component parts and accurate assembly thereof are required. Moreover, when the heat load of the evaporator or the rotation speed of the compressor is changed quickly, an increased amount of discharge gas flows into the crank chamber through a communication passage of the valve control mechanism due to a lag between the action of the valve control mechanism and the response of the external circuit including the compressor. As a result of the increased discharge gas flow, the efficiency of the compressor decreases and the durability of the compressor components is reduced.
The type of capacity adjustment described above using fluid communication between the discharge chamber and the crank chamber may be used in any type of compressor which uses a slanted plate or surface in the drive mechanism. For example, U.S. Pat. No. 4,664,604 to Terauchi discloses a swash plate type compressor. The swash plate, like the wobble plate, is disposed at a slant angle and drivingly couples the pistons to the drive source. However, while the wobble plate only nutates, the swash plate both nutates and rotates. The term slant plate type compressor will therefore be used herein to refer to any type of compressor, including wobble and swash plate types, which use a slanted plate or surface in the drive mechanism.